1. Field of the Invention
The present invention relates to a machine tool having a workpiece table for fixing workpieces to be machined, and a tool spindle with a fixture for tools used for machining the workpieces.
2. Related Prior Art
Machine tools of this kind are known from the prior art.
The known machine tools generally have a vertical-axis tool spindle arranged above a workpiece table on which the workpieces to be machined are fixed by clamping means known as fixture.
The tool spindle and the workpiece table are displaceable one relative to the other along three orthogonal axes so that the upside of the workpiece can be machined by a cutting operation in the known way.
During operation, one normally uses a coolant in order to cool the active or operating tool and the workpiece and to carry off the chips produced during the machining operation.
The use of such coolants leads to quite a number of problems. The coolant, which is collected in a chip pan together with the chips produced, must be purified and filtered, an operation that requires extensive handling. During this operation, the coolant evaporates and, due to its aggressive nature, may cause skin problems on the part of the service personnel, so that high disposal costs are connected with the use of coolant, in addition to the before-mentioned extensive handling.
It is known today that depending on the particular workpiece 10 to 25 percent of the production costs are due to the coolant. Aluminium, for example, gets spotty during machining as a result of the use of coolant and must be cleaned upon completion of the operation, and dried in a further step.
All in all, the use of coolant is connected with big problems so that one has tried for some time to operate known machine tools with a system called minimum-quantity lubrication, which as such is known in the art. In the case of such systems using minimum-quantity lubrication only one cubic centimeter of coolant is employed per hour; this can be technically controlled with the aid of novel tool geometries and coatings. However, problems arise in connection with the hot chips.
For, only 10 percent of the heat produced by the machining operation is stored in the workpiece, while the remaining 90 percent is absorbed by the chips produced. The chips fall onto surface areas, for example of carriages, where they lead to thermal deflections. By splashing the hot chips with coolant they are then cooled and, on the other hand, carried off directly into the chip pan.
The chips produced during machining with a minimum-quantity lubrication system or even during dry machining are clearly hotter than in cases where a coolant is used, and lead to clearly greater thermal deflections of the machine tool, that are difficult to control by control engineering means.
The hot chips heat up the machine structure, which leads to the before-mentioned accuracy problems but may also result in functional problems if the chips settle on the workpiece or parts of the machine. If such chips remain deposited on the guides of the carriages, they may be pressed into the guides during movement of the carriage and may damage the guides to an extent that the dimensional accuracy during displacement of the carriage may be impaired.
In view of the above it is an object of the present invention to reduce the before-mentioned problems connected with the chips in a machine tool of the kind mentioned at the outset by simple constructional means.
This object is achieved with a machine tool of the kind mentioned at the outset in that the machine tool uses an overhead tool pointing upwards when in its operating condition.
The object underlying the present invention is achieved in this way.
For, the inventors of the present invention have realized that by dropping the conventional machine concept and choosing an xe2x80x9coverhead machining systemxe2x80x9d, the chips are permitted to fall freely so that they can no longer deposit on the workpiece or the workpiece table and/or the latter""s guides, but can be carried off directly into the chip pan.
This novel machine concept now also permits dry machining or machining with minimum-quantity lubrication, the coolant being no longer required to carry off the chips produced. And since the hot chips can no longer deposit on the workpiece or in the area of the workpiece table, no thermal deflections will occur in this area either, so that the cooling effect of the coolant is also no longer required to the same extent as before.
To say it in other words: Due to the xe2x80x9coverhead machining systemxe2x80x9d the novel concept of the machine tool allows dry machining or machining with minimum-quantity lubrication, without the problems encountered in the prior art in connection with dry machining and/or minimum-quantity lubrication having the same disadvantageous effects. Due to the small quantity of coolant used the chips are no longer wetted with coolant so that they cannot stick to thexe2x80x94preferably verticalxe2x80x94structures of the new machine tool as they drop down xe2x80x9cfrom the workpiecexe2x80x9d. Rather, the dry chips drop directly into the chip pan, guided by suitable baffles. The chip pan, however, is heated up by the very hot chips to a much higher degree than in the case of the before-mentioned machine tool. The temperatures encountered in this connection can be controlled, for example, by insulating the chip pan thermally from the hot chips, whereby thermal deflections of the machine tool as such can be avoided.
According to a further improvement it is preferred if the tool spindle, with its tool-holding fixture pointing upwards, is seated on a mechanical translation system which is arranged, at least in part, below the workpiece table and which preferably comprises at least one lever mechanism mounted on a machine frame of the machine tool so that it can pivot about a vertical axis formed by a joint.
This feature on the one hand provides the advantage that the novel machine tool offers very high machining accuracy due to the fact that the spindle is arranged in the direct neighborhood of the mechanical translation system, below the workpiece table so that no cantilevers or the like are required. This, by first approximation, increases of course the risk that chips may get stuck on parts of the mechanical translation system. However, by designing the mechanical translation system as lever mechanism this risk is again clearly minimized as the mechanical translation system does not comprise any complex guides and carriages that extend along the three orthogonal axes one relative to the other; the movement of the tool spindle is effected through a pure lever mechanism, at least in the plane of the workpiece table, so that there remain only little surfaces of attack for the chips falling to the bottom.
It is preferred in this case if the lever mechanism comprises two vertically oriented plates that can be pivoted about the joint one relative to the other, one of the plates being connected with the tool spindle, the other one with the machine frame.
This feature initially provides constructional advantages, with respect to the machining accuracy, since the plates of the lever mechanism provide a very stiff mechanical translation system so that the tool spindle can be seated directly on the machine frame, via the lever mechanism, in considerably overhanging arrangement below and/or behind the workpiece table. It is preferred in this connection if two lever mechanisms are provided which are again mounted on the machine frame in vertically displaceable arrangement. One achieves in this way on the one hand very good stiffness and safety against tilting, while on the other hand the vertically extending guide offers only small surfaces of attack on which the chips produced may settle.
It is generally preferred if the joints are provided with caps to protect them from chips and if, preferably, the plates comprise chip-repellent surfaces on their upper narrow sides.
This feature also ensures, in a constructionally simple way, that the chips produced will not settle on the surfaces of the mechanical translation system, but will be effectively carried off into the chip pan without any additional aids. Although there remains a risk in the case of long chips that these may settle on the upper narrow sides of the plates of the lever mechanisms, the production of long chips can, however, be avoided by the use of a suitable tool geometry, as is generally known in the art.
It is generally preferred if the workpiece table comprises a device for fixing the workpieces, which is accessible from below, and preferably two horizontal and parallel supports, on which the device is arranged for pivotal movement about a horizontal axis.
This feature provides the advantage that the workpiece table is sort of open towards the bottom, in the area of the device, so that the free fall of the chips is not obstructed in any way. On the other hand, however, it also allows comfortable operation of the novel machine tool, as for changing the workpiece the device, which may be designed in the form of a cradle plate, only has to be pivoted by 180xc2x0 about the horizontal axis so that the finished workpiece comes to lie on top of the workpiece table. The operator can now remove the workpiece, and mount a new workpiece, without any difficulty.
One recognizes in this connection a big additional advantage of the new machine tool insofar as the operators are no longer hindered by the spindle when carrying out the work necessary for the workpiece change, since the spindle is now located beneath the workpiece table and no longerxe2x80x94as was usual heretoforexe2x80x94above the workpiece table. The novel machine tool therefore no longer presents the risk of injury, for example by hot chips, that may drop from the tool spindle later.
An additional advantage of the novel machine tool lies in the fact that it has a notably lower height because the space above the workpiece table heretofore used by the mechanical translation system of the tool spindle is no longer needed, the mechanical translation system being now accommodated in the previously unused space beneath the workpiece table. The novel machine tool is, thus, clearly compacter than known machine tools.
It is then preferred in this case if a chip baffle, extending obliquely in downward and forward direction, is arranged between the supports, with a chip collector funnel preferably arranged below the mechanical translation system for transferring any chips encountered into a chip pan.
The chip baffle and the chip collector funnel ensure very efficiently that the chips produced during the machining of workpieces are directed immediately into the chip pan, whereby the chips are prevented from penetrating into exposed parts of the mechanical translation system. This is so because due to its forward and downward inclination the chip baffle comes to lie between the upper end of the vertical guides and carriages, on which the lever mechanisms are displaced vertically, and the workpiece, i.e. the source of the chips encountered.
It is further generally preferred if a tool magazine with storage positions for tools is arranged on the machine frame and if the different storage positions can be directly addressed by the tool spindle for tool changes.
To provide a tool magazine in the working area of a machine tool has been known before, but as a rule tool changes are effected by additional tool changers that are equipped with grippers and swinging arms and capable of changing tools between the tool magazine and the tool-holding fixture of the spindle. The tool magazine is arranged in this case clearly above the workpiece table in order to prevent soiling of the available tools by chips flying around or by splashing coolant. The tool changer is required in this case for removing the tools from above and for inserting them into the spindle from below.
However, due to the xe2x80x9coverhead arrangementxe2x80x9d it is now possible for the tool spindle to address the individual storage positions of the tool magazine directly. The tool magazine may be arranged in this case above the plane of the workpiece table so that soiling of the available tools is prevented.
Consequently, the novel machine concept additionally makes it possible to do without the tool changers, without sacrificing the advantage provided by them according to the prior art, namely that the tools are not soiled in their storage positions.
Further advantages are evident from the description below of the appended drawings.
It is understood that the features mentioned above and those yet to be explained below can be used not only in the respective combinations indicated, but also in other combinations or in isolation, without leaving the context of the present invention.